The present invention relates to a method of transmitting a digital signal, a digital signal transmission and reception apparatus, and a digital signal transmission apparatus.
The SDH (Synchronous Digital Hierarchy) network of broad band that is based on the CCITT (Comite Consultatif Internationale Telegraphique et Telephonique) is constructed now. A demand for video transmission services of high quality has increased recently. Concurrently therewith, it is proposed to use the SDH network as a transmission line of such high-quality video transmission services (e.g., Collection of Speech Papers at the Meeting held on Spring 1992 by the Institute of Electronics, Information and Communication Engineers of Japan, Paper B-710).
FIGS. 6 and 10 of the accompanying drawings show examples of two kinds of formats of the transmission frame according to the conventional transmission method in a transmission line based on the synchronous digital hierarchy (SDH) network. FIG. 7 shows an example of an apparatus which transmits a video signal as an information signal by using the method shown in FIG. 6. FIG. 11 shows an example of an apparatus which similarly transmits the video signal by using the method shown in FIG. 10.
As shown in FIG. 6, there are provided a transmission line header 51 and an information signal header 52 which includes stuff control information. There is provided information signal data area 53 which includes a stuff word 54 (10-bit length). The information signal header, the information signal data area and a reserve area 57 for future expansion constitute an information signal frame 55 which simultaneously corresponds to a data area of a transmission line. The transmission header 51, the information signal header 52, the information signal data area 53, the stuff word 54 and the information signal frame 55 constitute a transmission line frame 56.
As shown in FIG. 7, there is provided an input terminal 1 to which a video signal is input as an information signal. The video signal applied to the input terminal 1 is converted by an analog-to-digital (A/D) converting unit 2 in the form of analog to digital signal. The output digital video signal from the A/D converting unit 2 is supplied to an information frame constructing unit 3. The output signal from the information signal frame constructing unit 3 is supplied to a transmission line frame constructing unit 4. The information signal frame constructing unit 3 and the transmission line frame constructing unit 4 constitute a transmission apparatus 32. There is shown a transmission line 5 whose transmission signal format is based on the above-mentioned synchronous digital hierarchy (SDH) network. The signal from the transmission apparatus 32 is supplied to a transmission line frame analyzing unit 6. The output signal from the transmission line frame analyzing unit 6 is supplied to an information signal frame analyzing unit 7. The transmission line frame analyzing unit 6 and the information signal frame analyzing unit 7 constitute a reception apparatus 33. The output signal from the reception apparatus 33 is supplied to a digital-to-analog (D/A) converting unit 8, in which it is converted from a digital to an analog signal. The analog video signal thus converted is delivered from an output terminal 9 as an information signal.
FIG. 8 shows an arrangement of the transmission apparatus 32 shown in FIG. 7. As shown in FIG. 8, there is provided an input terminal 10 to which a digitized information signal is input. The digitized information signal applied to the input terminal 10 is supplied to an information signal speed converting unit 11, in which it is converted in speed. The output signal from the information signal speed converting unit 11 is supplied to an information signal header synthesizing unit 12. Also, there is provided a stuff control unit 13. The information signal speed converting unit 11, the information signal header synthesizing unit 12 and the stuff control unit 13 constitute the information signal frame constructing unit 3. An interface signal speed converting unit 15 converts a speed of an information signal supplied thereto, and supplies the information signal thus converted to a transmission line header synthesizing unit 16. An interface signal clock generating unit 17 is formed of some suitable means, such as a subsidiary oscillator or the like. There is shown a transmission line frame control unit 18. A transmission line clock generating unit 19 is formed of an independent oscillator. The interface signal speed converting unit 15, the transmission line header synthesizing unit 16, the interface signal clock generating unit 17, the transmission line frame control unit 18 and the transmission line clock generating unit 19 constitute the transmission line frame constructing unit 4. The output of the transmission line frame constructing unit 4 is delivered from a transmission line output terminal 20.
FIG. 9 shows an arrangement of the reception apparatus 33 shown in FIG. 7. As shown in FIG. 9, there are provided a transmission line input terminal 21, a transmission line frame synchronization detection protecting unit 22 and a transmission line frame control unit 23. An interface signal speed converting unit 24 effects the speed conversion on the input signal to output an interface signal. An interface signal clock generating unit 25 is formed of some suitable means, such as a subsidiary oscillator or the like. The transmission line frame synchronization detection protecting unit 22, the transmission line frame control unit 23, the interface signal speed converting unit 24 and the interface signal clock generating unit 25 constitute the transmission line frame analyzing unit 6. There is shown a stuff control unit 28. An information signal speed converting unit 29 effects the speed conversion on the input signal to output an information signal. An information signal clock generating unit 30 is formed of some suitable means, such as a subsidiary oscillator or the like. The stuff control unit 28, the information signal speed converting unit 29 and the information signal clock generating unit 30 constitute the information signal frame analyzing unit 7. The output signal from the information signal frame analyzing unit 7 is delivered from a digital information output terminal 31.
As shown in FIG. 10, there is provided an information signal header 61 which includes frame synchronizing information of the information signal and stuff control information, though not shown. There is provided an information signal data area 62 which includes 2 words of stuff words (10-bit length) 63, 64. The information signal header 61, the information signal data area and the stuff words 63, 64 constitute a 250-byte information signal frame 65. There are provided a transmission line header 66 and a transmission line data area 67. The transmission line data area 67 includes therein a reserve area 69 that is used for future expansion. The information signal frame 65 is located in the transmission line data area 67 at its arbitrary position except the portion of the reserve area 69. The transmission line header 66 and the transmission line data area 67 constitute a transmission line frame 68.
A transmission apparatus shown in FIG. 11 is different from the transmission apparatus shown in FIG. 7 only in the arrangements of the transmission apparatus 32 and the reception apparatus 33. As shown in FIG. 12 which illustrates the arrangement of the transmission apparatus 32, there is shown an information signal frame control unit 14. As shown in FIG. 13 which illustrates the arrangement of the reception apparatus 33, there are provided an information signal frame synchronization detection protecting unit 26 and an information signal frame control unit 27. A remainder of the arrangements is fundamentally the same as those of the transmission apparatus shown in FIGS. 7, 8 and 9. Parts and elements identical to those of FIGS. 7, 8 and 9 are marked with the same reference characters and therefore need not be described in detail.
Operation of the transmission apparatus thus arranged will be described below. The transmission apparatus shown in FIGS. 7, 8 and 9 will be described initially. An analog information signal, such as a video signal or the like, is input to the information signal input terminal 1 and converted by the A/D converting unit 2 into a digital signal. Then, the digital video signal is input to the information signal frame constructing unit 3 in the transmission apparatus 32 from the digital information signal input terminal 10. In the information signal frame constructing unit 3, the stuff control unit 13 effects the speed difference detection and the stuff control, the information signal speed converting unit 11 effects the speed conversion to the interface speed and the stuff insertion and the information signal header synthesizing unit 12 effects the addition of the information signal header 52, such as the stuff control information or the like, respectively. The signal thus converted into the interface speed is input to the transmission line frame constructing unit 4. At that time, information signal frame control information, such as the header addition position or the like, and the interface signal clock are transmitted from the transmission line frame constructing unit 4 to the information signal frame constructing unit 3. In the transmission line frame constructing unit 4, the interface signal speed converting unit 15 effects the speed conversion to the transmission line speed and the transmission line header synthesizing unit 16 effects the addition of the transmission line header 51. Thus, a signal is transmitted from the transmission line output terminal 20 to the transmission line 5. The signal on the transmission line 5 is arranged as the transmission line frame 56 shown in FIG. 6 and then transmitted.
In the reception apparatus 33, the transmission line signal from the transmission line input terminal 21 is input to the transmission line frame analyzing unit 6, and the frame synchronization of the transmission line frame 56 is established by the transmission line frame synchronization detection protecting unit 22. The signal that had been converted by the interface signal speed converting unit 24 into the interface speed is input to the information signal frame analyzing unit 7 together with the frame control information generated by the transmission line frame control unit 22 and the interface signal clock generated by the interface signal clock generating unit 25. The information signal frame analyzing unit 7 separates the information signal header 52 by the frame control information, and the stuff control unit 28 effects the stuff control. Then, the digital information signal is restored by the information signal speed converting unit 29 and the information signal clock generating unit 30. The digital information signal is transmitted from the digital information signal output terminal 31 to the D/A converting unit 8. The D/A converting unit 8 restores the original analog information signal and outputs the same from the information signal output terminal 9.
The transmission apparatus shown in FIGS. 11, 12 and 14 will be described next. Initially, an analog information signal, such as a video signal or the like, is input to the information signal input terminal 1 and converted by the A/D converting unit 2 into a digital signal. Then the digital signal is input to the information signal frame constructing unit 3 in the transmission apparatus 32 from the digital information signal input terminal 10. In the information signal frame constructing unit 3, the stuff control unit 13 effects the speed difference detection and the control of the stuff insertion, the information signal speed converting unit 11 effects the speed conversion to the interface speed, the maintenance of the header area and the stuff insertion, and the information signal header synthesizing unit 12 effects the addition of the information signal header 61, such as the information signal frame synchronizing information, the stuff control information or the like, respectively. The signal thus converted into the interface speed is input to the transmission line frame constructing unit 4 under the condition that it constructs the information signal frame 65. At that time, while the interface signal clock is transmitted from the transmission line frame constructing unit 4 to the information signal frame constructing unit 3, information signal frame control information, such as header addition position information to the information signal header synthesizing unit 12, frame length information and intraframe data amount information to the stuff control unit 13, are generated by the information signal frame control unit 14. In the transmission line frame constructing unit 4, the interface signal speed converting unit 15 effects the speed conversion to the transmission line speed and the transmission line header synthesizing unit 16 effects the addition to the transmission line header 66, thereby a signal being transmitted to the transmission line 5 from the transmission line output terminal 20. The signal on the transmission line 5 is arranged as the transmission frame 58 shown in FIG. 10.
In the reception apparatus 33, the signal from the transmission line input terminal 21 is input to the transmission line frame analyzing unit 6, and a frame synchronization in the transmission frame 68 is established by the transmission line frame synchronizing detection protecting unit 22. The signal that was converted by the interface signal speed converting unit 24 into the interface speed is input to the information signal frame analyzing unit 7 together with the interface signal clock generated by the interface signal clock generating unit 25. In the information signal frame analyzing unit 7, the information signal frame synchronizing detection protecting unit 26 establishes the frame synchronization of the information signal frame 65 independently of the transmission line frame 68. The information signal header 61 is separated by the information signal frame control information generated by the information signal frame control unit 27, and the stuff is removed by using the stuff control information provided within the information signal header 61 under the control of the stuff control unit 28. Then, the digital information signal clock is restored by the information signal clock generating unit 30 and the header area and the stuff are removed by the information signal speed converting unit 29, thereby digital information signal data being restored. The digital information signal data thus restored is transmitted to the D/A converting unit 8 from the digital information signal output terminal 31. The D/A converting unit 8 restores the original analog information signal and outputs the same from the information signal output terminal 9.
As described above, according to the conventional transmission method and apparatus, in the example shown in FIGS. 6, 7, 8 and 9, the information signal header can be separated by establishing the frame synchronization of the transmission frame and the information signal can be restored by the stuff information obtained. Similarly, in the example shown in FIGS. 10, 11, 12 and 13, the frame synchronization of the information signal frame is established by the information signal frame synchronizing information provided within the information signal header, whereby the information signal header can be separated and the stuff information can be obtained, thus to restore the information signal.
According to the conventional transmission method, however, in the example shown in FIGS. 6, 7, 8 and 9, the information signal is transmitted at the frame unit of the transmission line so that a stuff ratio is uniquely determined and a residual jitter is uniquely determined by the frame arrangement of the transmission line. There is then the problem that the residual jitter is increased depending upon the combination of a transmission frame length, transmission rate and an information speed. By way of example, a characteristic curve B in FIG. 5 shows an input frequency fluctuation versus residual jitter characteristic obtained when a video signal that had been digitized at a sampling frequency 14.31818 MHz and a quantization bit of 10 bits is transmitted on the transmission line based on the synchronous digital hierarchy network as an information signal. A method of calculating a residual jitter characteristic is reported in a paper Vol. 58, No. 8 (1975), Part A, pp. 538 to 545 edited by the Institute of Electronics, Information and Communication Engineers of Japan. Study of recent reports reveals that the increase of such residual jitter exerts a bad influence on the phase characteristic of an information signal to be transmitted. Particularly, in the video transmission system, the residual jitter appears as noise caused when a phase of a color subcarrier is modulated, and deteriorates a picture quality (e.g., pp. 287 to 294, a paper Vol. 57, No. 4 (1974), Part A, edited by the Institute of Electronics, Information and Communication Engineers of Japan).
In the example shown in FIGS. 10, 11, 12 and 13, the information signal frame that is independent of the transmission frame is constructed and the stuff control is carried out at the unit of the information signal frame, whereby the increase of the residual jitter can be avoided regardless of the combination of the frame arrangement of the transmission line, the transmission rate and the information speed. By way of example, a characteristic curve A in FIG. 5 shows an input frequency fluctuation versus residual jitter characteristic obtained when a digital video signal having a sampling frequency 14.31818 MHz and a quantization bit of 10 bits is transmitted on the transmission line based on the synchronous digital hierarchy as an information signal.
In the example shown in FIGS. 10, 11, 12 and 13, because the position of the information signal frame within the transmission line frame is not fixed, the information signal frame synchronizing information within the information signal header must be generated by the information signal frame constructing unit 3 in the transmission apparatus 32. Also, the frame synchronization of the information signal frame must be established by capturing the information signal frame synchronizing information within the information signal header by the information signal frame analyzing unit 7 in the reception apparatus 33. There is then the problem that the scale of the required circuit is increased.
Furthermore, "A Development for High Quality Video Transmission System Based on SDH", ITEJ Technical Report Vol. 16, pp. 61-66, Oct. 1992 has proposed a transmission system based on the SDH. This previously-proposed transmission system uses a transmission frame (STM-1, VC-4 and C-4 bulk) based on the SDH and a frame formed of a video frame and an audio frame. This technical literature describes that areas within the VC-4 are assigned as shown in FIG. 14 and the following three systems are considered for the arrangement of the video frame considering (a) residual jitter characteristic, (b) a difference (capacity of vacant area) between a transmission rate and an information speed and (c) a frame synchronization method:
(A) A video frame that is not related to the VC-4 is constructed and a video frame synchronization on the reception side is established by hunting a frame pattern; PA1 (B) A video frame synchronized with the VC-4 is constructed and a video frame synchronization is made common to the frame synchronization of the VC-4; and PA1 (C) A video transmission that was assigned to the inside of the VC-4 per se is treated as a video frame.
The aforesaid technical literature has proposed as the system (B) a video frame shown in FIG. 15, i.e., a video frame having a frame length of 250 bytes in which a video transmission area of one line of the C-4 bulk is taken at the unit.